This invention is related to a process for improving the formability, weldability and surface appearance of zinc coated and zinc alloy coated sheet steel, and in particular, this invention is directed to improving the formability and weldability of electrogalvanized sheet steel. Zinc coated sheet steel is used for a variety of different automotive components. For example, hot-dip galvanized sheet steel is used in portions of the automobile where surface appearance is not important such as the underbody, door beams and trunk interiors. On the other hand, because of their high surface quality appearance, galvanneal, electrogalvanized and zinc alloy coated sheet steels tend to be used throughout the exterior portions of automobiles such as doors, hoods and deck lids, where a high gloss painted finish is important.
Zinc coated sheet steel products enjoy a major share of the automotive market because they have excellent resistance to corrosion and mechanical damage. However the protective zinc coatings are viewed, in some instances, as being unfavorable with respect to formability and weldability when compared to zinc alloy coatings.
Zinc coatings applied to sheet products tend to deform and gall during press forming operations. When the forming punch makes contact with the coated surface of the product, the coated surface galls and produces a buildup of zinc flakes within the die. The zinc flakes in turn cause defects in the surface appearance of the finished formed sheet product and, in order to overcome the problem, continuous downtime is required for maintenance and cleaning of the press forming dies.
Weldability of zinc coated sheet is also a problem. It is generally inferior to the weldability properties of zinc alloy coated or uncoated sheet steel. This is because the zinc coating melts during resistance welding and alloys with the copper in the electrode tip. The chemical reaction causes poor quality weld joints and reduces weld tip life.
The forming and welding difficulties encountered with zinc coated sheet steel is well known within the steelmaking industry. In the past, there have been various attempts to improve both the formability and weldability. One of the more significant solutions to the problem is to provide a layer on the outer surface of the protective zinc or zinc alloy coating which will improve the forming and welding properties.
U.S. Pat. No. 3,843,494 granted to Brown on Oct. 22, 1974 shows one such improvement. Brown discloses a process comprising the steps of applying on a ferrous metal substrate separate layers of metallic zinc and metallic iron, the outermost layer being a metallic iron layer which promotes the ease with which a plurality of said zinc coated ferrous substrates may be welded by resistance spot welding.
A further improvement in the art, directed more to surface appearance than weldability, is shown in U.S. Pat. No. 4,707,415. This patent teaches dipping zinc alloy coated sheet steel into an acidic oxidizing solution to electrochemically form a passive-state layer on the surface of the zinc alloy coating. The passive-state layer comprises at least one of oxides, hydroxides, and sulfides of zinc and nickel.
U.S. Pat. Nos. 4,957,594 and 5,203,986 teach forming a zinc oxide layer on the surface of zinc and zinc alloy steels to improve weldability. The 594 patent teaches adding an oxidizer to an acidic plating bath to form a zinc oxide or zinc hydroxide layer during the electroplating operation. Similarly, the 986 patent also teaches forming an oxide layer by using an oxidizer in an acidic plating bath, but with the addition of introducing a buffering agent into the bath to control the pH level.
Introducing various oxidizers and buffers into plating and coating baths to improve formability and weldability properties is not desirable from an operational viewpoint. Such additives tend to create complex, and sometimes unexpected, reactions which can lead to both environmental and product quality problems. For example, the addition of H.sub.2 O.sub.2 in a zinc sulphate plating bath can adversely impact on the morphology of the zinc plating and produce a coating unsuitable for finished automotive surfaces. Such additives also tend to reduce the efficiency of the coating line. Additionally, when nitrate or nitrite oxidizers are added to a plating bath, they may precipitate into complex compounds which are environmentally unsound and must be treated for proper disposal.
It has been discovered that the above problems can be avoided by using a post plating, or post coating, alkaline solution treatment to form a zinc oxide layer on the outer surface of a zinc or zinc alloy layer formed on a sheet steel product. This can be accomplished by applying an alkaline solution comprising an oxidizer to the surface of the zinc or zinc alloy layer, at a location separate from the plating or coating bath. The alkaline solution forms a suitable oxide layer on the surface of the zinc or zinc alloy layer, improves the formability and weldability, and avoids both environmental and product quality problems.